1. Field of the Invention
This invention relates to a high pressure discharge lamp lighting device comprising a super high pressure discharge lamp optimally used as the light source for a projector, etc. and a power supply device.
2. Description of Related Art
Lamps used as the light source of a projector, etc., are super high pressure discharge lamps where at least 0.2 mg/mm3 of mercury is enclosed in the interior of the discharge vessel and the pressure at the time of the lighting reaches at least 200 atm are utilized. It is known that, in such lamps, a projection is formed at the surface of the tip end of the electrodes when the lamp is lit and a stable lighting condition is maintained by holding the arc at these projections.
Now, if a high pressure discharge lamp remains lit in the same condition for a long time, a plurality of projections are formed and the tip end surface part of the electrodes may become uneven. It is known that, if the surface part of the electrode tip end is uneven, the discharge position becomes unstable and a reduction in brightness and flicker due to the movement of the arc.
As a solution for this problem, Japanese laid-open patent publication 2006-59790 describes a method of driving a discharge lamp wherein an alternating current with a frequency selected from a frequency range between 60 and 1000 Hz is supplied as the basic frequency at the steady state lighting and a lower frequency selected from the range between 5 and 200 Hz is supplied at specific intervals.
According to this technique, excessive tungsten adhered to the vicinity of the projections necessary for the discharge melts and the electrode tip ends are cleaned by supplying an alternating current such that the basic frequency being supplied at the time of the steady state lighting and a frequency being lower than the basic frequency are generated alternatingly; and because the shape of the projections can be maintained stable as the molten tungsten is supplied to the projections by means of the surface tension, is becomes possible to achieve a long lifetime of the electrodes.
Japanese laid-open patent publication 2007-280734 discloses a technique by means of which the lifetime of the electrodes is extended by changing the value of the frequency at the time of lighting. However, because the technique described in this patent publication constantly increases and decreases the frequency, over the course of time, there is the practical problem that an annoying noise is generated.
Further, Japanese patent 4089627 discloses a technique by means of which the wear of the electrodes is reduced and the lifetime of the lamp is extended by periodically changing the lamp power. However, with this technique, it is not possible to prevent brightness changes resulting from the power changes.